1. Field of the Invention
The invention relates to medical diagnosis and, more particularly, to a method of, and apparatus for, measuring venous blood volume changes in an animal body. The invention is especially useful for detecting the presence or absence of venous occlusions in the form of occult but potentially life-endangering blood clots or for detecting internal hemorrhage or other abnormalities which are accompanied by measurable venous blood volume changes.
2. Prior Art
The proper functioning of an animal body depends, among other factors, on a continual flow of blood through the blood vessels. In some patients, various pathological conditions may exist which impede this blood flow. For example, a blood clot may form in a vessel, typically a vein, and impede the flow, or internal hemorrhaging may occur which reduces the venous blood volume and may ultimately lead to shock and some times death.
Presently, various techniques are used to diagnose these and related conditions, but each technique has attendant disadvantages which frequently severely limit its reliability, its practicability, or both. For example, in the vitally important area of diagnosing the presence or absence of blood clots in a vein, the most common technique involves a physical examination of the patient for patent signs such as swelling, local tenderness, pain in the calf on dorsiflexion of the foot, distension of superficial veins, etc. The technique requires skill and experience in interpreting the observations made and is frequently inaccurate, especially in diagnosing the presence of small clots.
Other techniques for detecting blood clots involve the injection of x-ray opaque substances into the blood vessels followed by radiologic examination of the vessels, as is done in venography, or the injection of a radioisotope tracer into the blood stream followed by monitoring of the radioactive emission to determine whether there is an area of concentration indicating a blood clot. These techniques are more accurate than physical examination alone, but are both invasive and traumatic, and thus their use on many patients is frequently restricted. Further, the injected substances must be sterile and the patient carefully watched for adverse reactions. Although uncommon, serious complications of these procedures have been reported. In addition, they require the time of a physician skilled in their interpretation.
Ultrasonic techniques have also been utilized in which the frequency of an ultrasonic beam passing through the blood vessels of a patient is correlated with the flow through the blood vessels. These techniques require great skill on the part of the user in properly positioning the transducers and in interpreting the sounds elicited. Their accuracy is therefore dependent to a large extent on the skill and experience of the person taking the measurements and interpreting the results. Even in the hands of the most experienced observers, these techniques are sometimes inaccurate, although better than unaided physical examination.
Techniques for detecting internal hemorrhaging have relied largely on direct observation of a patient's condition and on monitoring selected vital signs such as blood pressure, etc. These techniques do not always give an unequivocal indication. Further, they often require the continued attention and observation of medical personnel, and thus are limited in application. Invasive techniques such as the insertion of a cannula into the blood vessels have sometimes been used, but such techniques present frequent danger of infection, among other disadvantages.
Recently, interest has evolved in the utilization of impedance measurements to study the arterial circulation. Blood is a good conductor of electricity and the electrical conductivity (which is the reciprocal of the impedance) through a blood vessel is proportional to the volume of blood within the vessel. Within the arteries, the blood volume, and thus the conductivity, fluctuates in accordance with the pumping action of the heart. If an artery is blocked by a clot, however, the blood flow from the heart is reduced and the fluctuations in blood volume and electrical conductivity are correspondingly reduced. Thus, the fluctuations of conductivity as the heart pumps blood into the arteries provide an indication of the presence or absence of a clot in an artery. However, this arterial pulse wave tends to disappear in the small blood vessels of the capillary bed and thus provides no information about the condition of the veins. Accordingly, impedance measurement techniques have not heretofore been utilized to detect the presence of blood clots or other occlusions in the veins.
Further, the data obtained from prior impedance measurements does not provide a direct indication of the symptoms sought but must frequently be further processed and carefully interpreted by skilled and experienced personnel before meaningful conclusions can be drawn. This limits the applicability of such techniques primarily to laboratory investigations and, for all practical purposes, preclude their utilization in the physician's office, or in general hospital care, or in the operating room.
Another parameter of interest to the physician is the arterial blood flow rate. This provides important information relating to the condition of the heart, the tone of the arterial walls, and, indirectly, the blood volume, among other factors. Estimates of the blood flow rate in a subject have heretofore been made my numerous techniques, but none so far have provided sufficient accuracy or have been sufficiently simple and reliable in use as to lead to their acceptance as general diagnostic techniques.